In the 1950s, telecommunications companies began to develop high bandwidth digital communications technologies in order to allow more phone calls to be simultaneously transmitted over copper wire. The first digital transmission carrier, called T1, was developed by AT&T in 1956 and is still in use today. A T1 line is capable of transmitting 1.544 Megabits per second (Mbps). Originally utilized to connect telephone central offices, in the early 1980s T1 lines began to be utilized in the local loop.
The local loop is often thought of as the connection between a local telecommunications office and an end-user. The "end-user" could be an actual customer of telephone service, a bandwidth reseller such as an Internet service provider, or even a site maintained for the convenience of a telecommunications company. Although the local loop is commonly referred to as the "last mile," local loop lengths in the United States are more typically about 2.5 miles, and some local loop implementations having a maximum range of almost 50,000 feet.
Digital transmission carriers such as T1 are usually "channelized" into multiple channels using Time Division Multiplex (TDM) technology. TDM channels are created by a multiplexer that divides a digital carrier into separate, individual time segments. Each time segment is allocated for the exclusive use of a single channel. The standard T1 line is divided in this manner into 24 separate channels. Each channel transmits 8 bits of digital data before the next channel begins transmitting. Since every channel sends 8 bits down the T1 line in turn, a series of 192 bits (8 bits times 24 channels) is created before the process can repeat. Before each series of bits, the multiplexer adds an additional bit called the framing bit. Thus, data on a T1 line is sent in 193 bit long "frames." These frames are transmitted about 8,000 times per second.
Each channel in a T1 line is called a DS-0 channel. Similarly, the total T1 line is often referred to as a DS-1 line. Thus, there are 24 DS-0 channels in a DS-1 line. Each DS-0 channel transmits at 64 k bps. This transmission speed is the ideal bandwidth for voice communication, since voice communication is generally sampled and digitally converted into 8 bit words 8,000 times per second. In addition to serving voice communication, these DS-0 channels are commonly used for digital data communication.
The individual DS-0 channels can be operated in either a "switched" or "dedicated" fashion. Switched data channels allow the communication on the channel to be switched on and off. Voice communication is an example of switched data, in that there are times when the voice channel is active or "off-hook," and other times when a voice channel is inactive or "on-hook." Data communication can also operate in a switched fashion, sometimes actively communicating data and other times being inactive.
In order for a switched data channel to be switched on and off, it is necessary to signal the current status of the communication. In a voice channel, for example, it is necessary to indicate when a telephone receiver is picked up to place a phone call (signaled by an off-hook status indication), and to indicate when a local line should start ringing.
In contrast, a dedicated communication channel does not transmit status information and is always active. Although a dedicated channel may only be transmitting useful information at specific times, it does not ever become inactive.
Another important aspect of channelized digital transmission carriers is the possibility of combining multiple channels to obtain a higher bandwidth digital data path. For instance, three DS-0 channels can be combined into a single 192 k bps data communications path. Techniques for combining separate channels into a single, higher bandwidth digital communications path are well-known in the prior art.
It is common to have switched and unswitched data appearing simultaneously on the same channelized communication link. For example, a T1 to an office could be utilized to carry both voice communications over switched data channels and computer communications with the Internet over dedicated data channels. Traditionally, some DS-0 channels in the T1 line would be dedicated to carrying the switched, voice communications, while other DS-0 channels would carry the unswitched data communications.
Unfortunately, this fixed allocation of bandwidth on a local loop T1 line wastes bandwidth, since the switched DS-0 channels carry no data when they are idle. A better approach is to dynamically allocate the bandwidth on an as-needed basis. With dynamic bandwidth allocation, the inactive voice channels can be utilized to handle unswitched data communications when no voice calls are active, and yet would be available for voice communications when a signal to make the voice channel active is received.
The basic idea of allowing the same data channels to be used for both switched and unswitched communication is not new. One approach to doing so is implemented through Asynchronous Transfer Mode (ATM) technology. This technology is able to successfully provide and manage bandwidth for voice, video, and data applications. To accomplish this task, ATM utilizes "cell relay" techniques instead of relying on data channels created by time division multiplexing. In cell relay, each communications task, whether data, voice, or video, is divided into fixed size packets, or "cells," that contain a small amount of data and header information to direct the cell. Each cell is then transmitted with all other cells across the same communications path, and is directed toward its destination by the header information. Once the cells arrive at their destination, the communication is then reconstructed. While ATM may be the best solution for large-scale bandwidth-management problems, it is overly complex, too resource intensive, and too expensive for handling variable bandwidth assignments on the local loop.
A better approach is to keep the DS-0 channels created via time division multiplexing, and instead develop simpler techniques of dynamic bandwidth allocation. Unfortunately, the currently known prior art methods utilizing this approach fail to provide bandwidth allocation in a simple yet effective manner.
For instance, in U.S. Pat. No. 4,763,321 issued to Rozenblit and assigned to Bell Communications Research, Inc., a method for handling variable bandwidth allocation by changing the allocation of DS-0 channels is presented. This invention relates to Distributed Burst Switching Systems (DBSS), a system that uses virtual circuits in the manner of ATM, X.25 and Frame Relay. However, DBSS passes the frames containing the data through standard DS-0 channels. In standard DBSS, no virtual circuit can utilize more than one DS-0 channel, hence limiting transmission speeds on a virtual circuit to no more than 64 k bps. The Rozenblit invention allows a single virtual circuit to utilize more than one DS-0 channel. To accomplish this, no packets are transmitted between two nodes in a link until a 32 bit header is passed to the next node identifying the virtual circuit and specifying the number of DS-0 channels to be utilized for the virtual circuit. When a transmission between two nodes is completed, the transmitting node sends a 32-bit flag concluding the communication. When the ending flag is received, the DS-0 channels that had been utilized for the communication are freed up for use in another transmission. Unfortunately, the Rozenblit invention suffers from the same basic problems as the ATM technique, in that it imposes needless complexity and overhead on the relatively straight-forward situation of dynamic bandwidth allocation on the local loop.
Another bandwidth allocation scheme is revealed in U.S. Pat. No. 4,383,315, issued to Torng and assigned to Bell Telephone Laboratories. This invention is intended for use in a loop transmission system, where multiple nodes communicate by passing transmissions on to the next node in the loop. This approach modifies the content of the communication link to indicate the state of a channel. A key aspect in this invention is the process of deciding when to seize an idle time slot, given that other nodes present in the loop may also wish to use the time slot. Unfortunately, this application has little direct application to the allocation of bandwidth on the local loop. Unlike a loop transmission system, a local loop has only two nodes, and communicates over standard DS-0 channels. In addition, the Torng invention suffers in that it utilizes a type of collision detection, in which data messages can be overwritten before receipt by the intended node, and overcomes this problem by incorporating statistically based delays into the transmission of data. These delays prevent full utilization of available bandwidth, and are unnecessary in the local loop environment.
A third prior art approach to dynamic bandwidth allocation could be used on the local loop. In U.S. Pat. No. 5,467,344, issued to Solomon and assigned to Ascom Timeplex Trading AG, a system is disclosed specifically for changing bandwidth allocation across a T1 transmission line. In this disclosure, a method is described for using "pad" codes to fill data channels in transition. When a DS-0 channel on the T1 line is to be reallocated, these pad characters fill the soon-to-be reallocated DS-0 channel while a separate reconfiguration message is sent and confirmed on a different channel. Once the reallocation message is confirmed by the remote node, the reallocated DS-0 channel begins to carry live data. This method is used to avoid having to synchronize the switching of the bandwidth at each end of the link. As a result, this method is useful in long-haul environments where frame order and multiframing may be lost. A disadvantage to this approach is that special hardware is required that can eliminate the pad codes and encode and decode data to avoid spurious pad codes that might otherwise appear in a data stream. These disadvantages occur because the Solomon technique is not narrowly suited to the local loop environment, but instead is generally applicable to remote communication over channelized digital lines. This results in needless complexity, cost, and bandwidth overhead.
A final approach in the prior art is the utilization of dedicated signaling channels, such as that used with ISDN. In Basic Rate Interface ISDN, or BRI, two DS-0 channels of 64 k bps bandwidth (referred to as B Channels) are combined with a 16 k bps signaling channel (referred to as the D Channel). In Primary Rate Interface ISDN, or PRI, twenty-three DS-0 channels of 64 k bps bandwidth (B Channels) are combined with one 64 k bps signaling channel (the D Channel).
ISDN technology can be used to provide dynamic bandwidth allocation between switched data and unswitched data on the local loop. By taking advantage of the dedicated signaling channel, ISDN routers (such as the XpressConnect 5242i available from Gandolph Technologies Inc. of Nepean, Ontario, Canada) can handle dynamic bandwidth allocation. When used with Internet connection protocols such as the PPP Bandwidth Allocation Control Protocol (BACP) and the PPP Multilink specification (RFC 1717), also known as Multiport Protocol (MP), this type of ISDN router dynamically allocates bandwidth between a single unswitched data path (the PPP Internet connection) and a switched data path (a voice call). When a phone receiver is picked up, the bandwidth allocated to the PPP data path is reduced by 64 k bps and a DS-0 channel is available for the voice communication. When the voice call is over, the DS-0 channel previously carrying the voice call is reallocated to the PPP data path.
The primary disadvantage of ISDN variable bandwidth allocation on the local loop is that the D channel must be dedicated to handling signaling. Although the D channel can be used separately to handle other data tasks, it can not be utilized fully as part of the unswitched data path or as a switched data path. In addition, expensive ISDN technology is required to implement this technique. Although ISDN is often considered to have a bright future, few parties have invested heavily in ISDN equipment.
This invention addresses these problems in the prior art by providing a simple, non-intrusive mechanism for dynamically reallocating digital communication channels between switched and unswitched data in the local loop. The invention allows bandwidth assigned to switched channels to be reassigned to expand the bandwidth of an unswitched digital data path when said switched channel is inactive. Whenever a switched channel becomes active, the bandwidth would be returned to the switched channel. This is accomplished without using the complex procedure of combining switched and unswitched data on the same channel through the use of cells. In addition, the invention does not need to utilize complex and bandwidth intensive reconfiguration messages or padding characters. Instead, the invention is able to utilize the signaling already associated with a switched data channel to determine the timing of dynamic bandwidth switching.